dt-bindings: timer: Move snps,dw-apb-timer DT schema from rtc

dt-bindings: timer: Move snps,dw-apb-timer DT schema from rtc

This binding file doesn't belong to the rtc seeing it's a pure timer
with no rtc facilities like days/months/years counting and alarms.
So move the YAML-file to the Documentation/devicetree/bindings/timer/
directory.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200521204818.25436-3-Sergey.Semin@baikalelectronics.ru